Simultaneous online monitoring of inorganic compounds in aerosols and gases in an industrialized area

The automatic MARGA (monitor for aerosols and gases in ambient air) sampling system was used to measure the inorganic ions Cl–, NO3–, SO42–, Na+, NH4+, K+, Mg2+ and Ca2+ in the PM2.5 aerosol phase and the corresponding inorganic gases HCl, HNO2, SO2, HNO3 and NH3 present in the gas phase. Samples were collected and analyzed hourly for 3 months between April and June, 2011, from a sampling site in Singapore close to a heavy industrial area containing extensive petrochemical refineries. The data (hourly and daily average) were analyzed, compared and discussed based on the ratios of HNO2/HNO3 and NH3/NH4+, the levels of nitrate and sulfate, the total nitrogen, the distribution of particulate matter and gaseous compounds, and the acidity of the aerosols. SO2 was the most abundant gas that appeared in an order of magnitude higher concentration than the other trace gases, and correspondingly SO42– was found to be at least 3–10 times higher than other anionic aerosol species. The concentration of major ions in aerosol samples and the related gaseous compounds followed the order of: SO42– > NH4+ > NO3– > K+ > Na+ > Cl– > Ca2+ > Mg2+ and SO2 > NH3 > HNO2 > HNO3, respectively. The maximum values for many of the target analytes occurred during the hazy period in May when there was significant contamination from regional fires. The elevated levels of HNO2 compared to HNO3 and high levels of HNO3 were rationalized based on artifacts in the denuder sampling methodology.